An apparatus for detection of leaks in pressurized pipelines which utilizes a large pressure vessel and a small measurement vessel. The measurement vessel magnifies level changes during leak detection tests. The apparatus is connected to a pipeline through the measurement vessel. The entire system can be filled with liquid from the pipeline by opening a valve between the measurement and pressure vessels. Leak detection tests are conducted by measuring changes in volume with the measurement vessel over time while the pressure over the liquid in the pressure vessel and measurement vessel is maintained approximately constant. During tests, liquid communication between the measurement vessel and pressure vessel is prevented by closing the valve between them, but vapor communication between the vessels is permitted.